The present invention relates to a phase correction method and an MRI (Magnetic Resonance Imaging) system, more specifically, to a phase correction method and an MRI system which can correct motion phase errors and simplify the pulse sequence.
FIG. 18 shows a basic example of a pulse sequence of a multishot diffusion enhancement EPI (Echo Planar Imaging) method.
In the pulse sequence, an excitation pulse RF90 and a slice gradient SG90 are applied. An MPG (Motion Probing Gradient) pulse MPG is then applied. An inverting RF pulse RF180 and a slice gradient SG180 are applied. An MPG pulse MPG is then applied. A phase encode gradient pdn is applied. Data collecting read gradients r1 . . . rM which are alternately inverted to be positive or negative are applied continuously, phase encode gradients p2, . . . , pM are applied at inverting, and they are sampled by being timed to successively focus the first echo el to the Mth echo eM so as to collect data F (n, 1), . . . , F (n, M) corresponding to the echoes el, . . . , eM. This is repeated for nxc3x971, . . . , N while changing the magnitude of the phase encode gradient pdn, to thereby collect data F (1, 1) to F (N, M) for filling the k space. This is called N shot and M echo. A number n given to a shot in the order of execution time is called a shot number. A number given to the echo of the echo train of a certain shot in the order of focusing time is called an echo number.
FIG. 19 is a schematic diagram showing collection trajectories of data F (1, 1) to F (N, M) in a k space KS, where N=4 and M=4.
When the k space KS is divided in the phase encode axis direction from the first line to the Nxc3x97Mth line (or to the 16th line in FIG. 19), phase encodes pdn, p2, . . . , pM are applied so as to collect data F (n, m) of the (n+(mxe2x88x921) N) th line by the Mth echo of the nth shot.
As shown in FIG. 20, the k space KS can be successively divided into blocks from the first echo block filled by data F (n, 1) obtained from the first echo of the first to the Nth shot, to the Mth echo block (M=4 in FIG. 20) filled by data F (n, M) obtained from the Mth echo of the shots.
Phase errors which are a problem in the pulse sequence of the multishot diffusion enhancement EPI method include a motion phase error caused by motion (for example, a pulse of a brain) and a magnetic field inhomogeneity error caused by magnetic field inhomogeneity.
The magnitude of the motion phase error periodically fluctuates in synchronization with, e.g., a pulse. In other words, the motion phase error fluctuates by the magnitude which cannot be ignored at relatively long time intervals between the shots. The motion phase error is changed only by the degree which can be ignored within a relatively short time such as an echo train of one shot. The motion phase errors of the same shot number n can thus be regarded as the same magnitude although the echo number m is different.
The magnitude of the magnetic field inhomogeneity phase error is increased in proportion to time from the excitation pulse RF90. In other words, the magnetic field inhomogeneity phase error is increased in proportion to the echo number m. Time to the echo number m from the excitation pulse RF90 is the same or is slightly different (there are the cases that the time is the same and that the time is slightly different) although the shot number n is different. The magnetic field inhomogeneity phase error of the echo number m can be regarded as the same magnitude although the shot number n is different.
FIG. 21 is an explanatory view showing phase errors of data F (n, m) according to a basic example of the pulse sequence of the multishot diffusion enhancement EPI method of FIG. 18.
After the motion phase error is synthesized with the magnetic field inhomogeneity phase error, the phase errors are stepwise and periodical, causing ghost artifacts.
Like the pulse sequence shown in FIG. 22, a navigation phase encode gradient Nr is applied before the phase encode gradient pdn of the pulse sequence (the basic example) of FIG. 18 so as to focus a navigation echo Ne and to collect correcting data H (n) from the navigation echo Ne.
A phase difference between the correcting data H (n) of the shots represents a difference between the motion phase errors of the shots. Based on the correcting data H (n), imaging data F (n, 1) to F (n, m) of the same shot are phase corrected to correct the motion phase errors.
FIG. 23 is an explanatory view showing phase errors of data Fxe2x80x2 (n, m) after phase correction.
Imaging data F (1, 1) to F (4, 4) are phase corrected (indicated by the black-headed arrows) so that the phase of correcting data H (1) of the first shot is matched with the phase of correcting data H (2) of the second shot to the phase of correcting data H (4) of the fourth shot. This can correct the motion phase errors and can suppress ghost artifacts as compared with the case of FIG. 23.
Since the magnetic field inhomogeneity phase errors remain, a phase difference is caused between the echo blocks. The ghost artifacts cannot be removed completely.
The pulse sequence of FIG. 18, because of the motion phase errors and the magnetic field inhomogeneity phase errors, has a problem of causing ghost artifacts.
The pulse sequence of FIG. 22, because of the magnetic field inhomogeneity phase errors, has a problem that ghost artifacts cannot be removed completely. The navigation echo Ne and the imaging echoes from e1 to em are independent. The pulse sequence is complex and the control is thus complicated.
Therefore, a first object of the present invention is to provide a phase correction method and an MRI system which can correct motion phase errors and can simplify the pulse sequence.
In addition to the first object, a second object of the present invention is to provide a phase correction method and an MRI system which can correct magnetic field inhomogeneity phase errors.
In a first aspect, the present invention provides a phase correction method including: repeating by N shots a pulse sequence in which when a k space is divided in the phase encode axis direction from the first line to the N=Mth (N and M are a natural number of 2 or more) line, data collecting read gradients are applied while inverting so as to focus M-piece imaging echoes per inverting RF pulse and to focus one or more navigation echoes as an echo train continuous to the M-piece imaging echoes before the M-piece imaging echoes and an MPG pulse is applied before and after the inverting pulse; collecting diffusion enhancement imaging data for filling the k space from the imaging echo; collecting correcting data from the navigation echo; and phase correcting the imaging data based on the correcting data.
In the phase correction method of the first aspect, since the correcting data collected for each of the shots corrects the phase of the imaging data of the same shot, the motion phase errors can be corrected. In addition; the navigation echo is focused so as to be an echo train continuous to the imaging echo. The applying pattern of data collecting read gradients can be simplified and the pulse sequence can be simplified.
In a second aspect, the present invention provides the phase correction method thus constructed, wherein two or more navigation echoes are provided to one shot, and the polarity of the data collecting read gradient corresponding to imaging data is matched with the polarity of the data collecting read gradient corresponding to correcting data for use in phase correction of the imaging data.
In the phase correction method of the second aspect, the polarity of the data collecting read gradient corresponding to imaging data is matched with the polarity of the data collecting read gradient corresponding to correcting data for use in phase correction of the imaging data. Proper correction can thus be performed as compared with the case that the polarities are different. In addition, two or more navigation echoes and imaging echoes successively correspond with each other for phase correction. A difference can be eliminated with the same number of echo blocks as the number of the navigation echoes as a unit.
In a third aspect, the present invention provides the phase correction method thus constructed, wherein two or more navigation echoes are provided to one shot, a correction amount of a motion phase error is determined from the phase of the navigation echo of the shots, and a correction amount of a magnetic field inhomogeneity phase error is determined from a phase difference between the navigation echoes of the shots.
In the phase correction method of the third aspect, each correcting data is collected from two or more navigation echoes, and a correction amount of a motion phase error and a correction amount of a magnetic field inhomogeneity phase error are determined. Both the motion phase error and the magnetic field inhomogeneity phase error can thus be corrected.
In a fourth aspect, the present invention provides the phase correction method thus constructed, wherein the imaging data is phase corrected so that the phase errors of the imaging data are smoothly changed in the phase encode axis direction from the first line to the Nxc3x97Mth line.
In the phase correction method of the fourth aspect, the phase errors of the imaging data are smoothly changed in the phase encode axis direction. Ghost artifacts can thus be removed.
In a fifth aspect, the present invention provides the phase correction method thus constructed, wherein the imaging data is phase corrected so that the phase errors of the imaging data are the same in the phase encode axis direction from the first line to the Nxc3x97Mth line.
In the phase correction method of the fifth aspect, the phase errors of the imaging data are the same in the phase encode axis direction. Thus, ghost artifacts can be removed and shifting of an image is suppressed.
In a sixth aspect, the present invention provides the phase correction method thus constructed, wherein one navigation echo is provided to one shot, and a correction amount of a motion phase error is determined from the phase of the navigation echo of the shots.
In the phase correction method of the sixth aspect, correcting data is collected from one navigation echo so as to determine a correction amount of a motion phase error. The motion phase error can thus be corrected. In addition, the pulse sequence can be simplified.
In a seventh aspect, the present invention provides a phase correction method including: repeating by N shots a pulse sequence in which when a k space is divided in the phase encode axis direction from the first line to the Nxc3x97Mth (N and M are a natural number of 2 or more) line, data collecting read gradients are applied while inverting so as to focus M-piece imaging echoes per inverting RF pulse and to focus one or more navigation echoes as an echo train continuous to the M-piece imaging echoes before the M-piece imaging echoes and an MPG pulse is applied before and after the inverting pulse; collecting diffusion enhancement imaging data for filling the k space from the imaging echo; collecting correcting data from the navigation echo; collecting referring data corresponding to the imaging data and correcting data from a referring echo focused by executing a referring pulse sequence omitting the phase encode gradient and MPG pulse from the pulse sequence; phase correcting the imaging data based on the corresponding referring data; phase correcting the correcting data based on the corresponding referring data; and phase correcting imaging data phasexe2x80x94corrected by the referring data based on correcting data phasexe2x80x94corrected by the referring data.
In the phase correction method of the seventh aspect, referring data collected by executing a referring pulse sequence phasexe2x80x94corrects imaging data and correcting data. Phase errors caused by the magnetic field inhomogeneity, system, and shifting of the echo center can thus be corrected. Further, since the correcting data collected for each of the shots corrects the phase of the imaging data of the same shot, the motion phase errors can be corrected. In other words, both the motion phase error and the magnetic field inhomogeneity phase error can be corrected. In addition, the navigation echo is focused so as to be an echo train continuous to the imaging echo. The applying pattern of data collecting read gradients can be simplified and the pulse sequence can be simplified.
In an eighth aspect, the present invention provides the phase correction method thus constructed, wherein two or more navigation echoes are provided to one shot, and the polarity of the data collecting read gradient corresponding to imaging data is matched with the polarity of the data collecting read gradient corresponding to correcting data for use in phase correction of the imaging data.
In the phase correction method of the eighth aspect, the polarity of the data collecting read gradient corresponding to imaging data is matched with the polarity of the data collecting read gradient corresponding to correcting data for use in phase correction of the imaging data. Proper correction can thus be performed as compared with the case that the polarities are different.
In a ninth aspect, the present invention provides the phase correction method thus constructed, wherein the imaging data is phase corrected so that the phase errors of the imaging data are the same in the phase encode axis direction from the first line to the Nxc3x97Mth line.
In the phase correction method of the ninth aspect, the phase errors of the imaging data are the same in the phase encode axis direction. Thus, ghost artifacts can be removed and shifting of an image is suppressed.
In a tenth aspect, the present invention provides the phase correction method thus constructed, wherein one navigation echo is provided to one shot, and a correction amount of a motion phase error is determined from the phase of the navigation echo of the shots.
In the phase correction method of the tenth aspect, since one navigation echo is provided, the pulse sequence can be simplified.
In an eleventh aspect, the present invention provides an MRI system including: RF pulse transmitting means; gradient pulse application means; NMR signal receiving means; data collection control means for controlling those means, repeating by N shots a pulse sequence in which when a k space is divided in the phase encode axis direction from the first line to the Nxc3x97Mth (N and M are a natural number of 2 or more) line, data collecting read gradients are applied while inverting so as to focus M-piece imaging echoes per inverting RF pulse and to focus one or more navigation echoes as an echo train continuous to the M-piece imaging echoes before the M-piece imaging echoes and an MPG pulse is applied before and after the inverting pulse, collecting diffusion enhancement imaging data for filling the k space from the imaging echo, and collecting correcting data from the navigation echo; correction arithmetic operation means for phase correcting the imaging data based on the correcting data; and reconstruction arithmetic operation means for reconstructing an image from the imaging data after correction arithmetic operation.
The MRI system of the eleventh aspect can preferably execute the phase correction method of the first aspect.
In a twelfth aspect, the present invention provides the MRI system thus constructed, wherein the data collection control means provides two or more navigation echoes to one shot, and the correction arithmetic operation means matches the polarity of the data collecting read gradient corresponding to imaging data with the polarity of the data collecting read gradient corresponding to correcting data for use in phase correction of the imaging data.
The MRI system of the twelfth aspect can preferably execute the phase correction method of the second aspect.
In a thirteenth aspect, the present invention provides the MRI system thus constructed, wherein the data collection control means provides two or more navigation echoes to one shot, and the correction arithmetic operation means determines a correction amount of a motion phase error from the phase of the navigation echo of the shots and determines a correction amount of a magnetic field inhomogeneity phase error from a phase difference between the navigation echoes of the shots.
The MRI system of the thirteenth aspect can preferably execute the phase correction method of the third aspect.
In a fourteenth aspect, the present invention provides the MRI system thus constructed, wherein the correction arithmetic operation means phase corrects the imaging data so that the phase errors of the imaging data are smoothly changed in the phase encode axis direction from the first line to the Nxc3x97Mth line.
The MRI system of the fourteenth aspect can preferably execute the phase correction method of the fourth aspect.
In a fifteenth aspect, the present invention provides the MRI system thus constructed, wherein the correction arithmetic operation means phase corrects the imaging data so that the phase errors of the imaging data are the same in the phase encode axis direction from the first line to the Nxc3x97Mth line.
The MRI system of the fifteenth aspect can preferably execute the phase correction method of the fifth, aspect.
In a sixteenth aspect, the present invention provides the MRI system thus constructed, wherein the data collection control means provides one navigation echo to one shot, and the correction arithmetic operation means determines a correction amount of a motion phase error from the phase of the navigation echo of the shots.
The MRI system of the sixteenth aspect can preferably execute the phase correction method of the sixth aspect.
In a seventeenth aspect, the present invention provides an MRI system including: RF pulse transmitting means; gradient pulse application means; NMR signal receiving means; data collection control means for controlling those means, repeating by N shots a pulse sequence in which when a k space is divided in the phase encode axis direction from the first line to the Nxc3x97Mth (N and M are a natural number of 2 or more) line, data collecting read gradients are applied while inverting so as to focus M-piece imaging echoes per inverting RF pulse and to focus one or more navigation echoes as an echo train continuous to the M-piece imaging echoes before the M-piece imaging echoes and an MPG pulse is applied before and after the inverting pulse, collecting diffusion enhancement imaging data for filling the k space from the imaging echo, collecting correcting data from the navigation echo, and collecting referring data corresponding to the imaging data and correcting data from a referring echo focused by executing a referring pulse sequence omitting the phase encode gradient and MPG pulse from the pulse sequence; correction arithmetic operation means for phase correcting the imaging data based on the corresponding referring data, phase correcting the correcting data based on the corresponding referring data, and phase correcting imaging data phasexe2x80x94corrected by the referring data based on correcting data phasexe2x80x94corrected by the referring data; and reconstruction arithmetic operation means for reconstructing an image from the imaging data after correction arithmetic operation.
The MRI system of the seventeenth aspect can preferably execute the phase correction method of the seventh aspect.
In an eighteenth aspect, the present invention provides the MRI system thus constructed, wherein the data collection control means provides two or more navigation echoes to one shot, and the correction arithmetic operation means matches the polarity of the data collecting read gradient corresponding to imaging data with the polarity of the data collecting read gradient corresponding to correcting data for use in phase correction of the imaging data.
The MRI system of the eighteenth aspect can preferably execute the phase correction method of the eighth aspect.
In a nineteenth aspect, the present invention provides the MRI system thus constructed, wherein the data collection control means provides one navigation echo to one shot so as to determine a correction amount of a motion phase error from the phase of the navigation echo of the shots.
The MRI system of the nineteenth aspect can preferably execute the phase correction method of the ninth aspect.
According to the phase correction method and the MRI system of the present invention, motion phase errors can be corrected and the pulse sequence can be simplified. Magnetic field inhomogeneity phase errors can be also corrected.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.